1. Field of the Invention
The present invention relates to a dielectric filter, and more particularly to a dielectric filter having a plurality of dielectric resonators on a single dielectric block.
2. Description of the Related Art
A previously known dielectric filter of this kind is shown, for example, in FIG. 9. In FIG. 9, a dielectric filter 1 has a plurality of through-holes 3 which cut through a rectangular box-shaped dielectric block 2 from the top surface to the bottom surface viewed from FIG. 9. On the internal surfaces of the through-holes 3, internal conductors 4 are formed respectively. An external conductor 5 is formed on the external surface of the dielectric block 2 except for the top surface. The through-holes 3, together with the external conductor 5 and the dielectric block 2, form dielectric resonators 6, respectively, with 1/4-wavelength, using the top and bottom surfaces of the dielectric block 2 as an open surface and a short surface respectively. The dielectric resonators 6 are electromagnetically coupled with each other to form a band-pass type filter.
On the top surface of the dielectric block 2, a pattern electrode 7 is formed to obtain a coupling capacitance Ck between the dielectric resonators 6 adjoining each other and to also adjust a stray capacitance Cs. Each of the pattern electrodes 7 is electrically coupled to one of the internal conductors 4, and the pattern electrodes 7 adjoining each other are separated across a gap g1 formed therebetween. On both ends of the top of the dielectric block 2, an input-pattern electrode 8 and an output-pattern electrode 9 are formed which are separated from the pattern electrodes 7 located at both ends across a gap g2 and a gap g3, respectively.
A dielectric filter is usually widely used for a filter of microwave band telecommunication equipment, and these apparatuses are being miniaturized each year. However, in a conventional dielectric filter, as shown in FIG. 9, with the size of the dielectric block 2 being reduced by miniaturization, an area of the pattern electrode 7 and a gap g1 between the pattern electrodes 7 adjoining each other are also reduced. Accordingly, when the pattern electrode 7 is printed in a process of manufacturing the dielectric filter 1, there has been a problem that bleeding associated with printing affects the coupling capacitance Ck and the stray capacitance Cs so as to greatly change them, causing fluctuation in characteristics of the dielectric filter 1.
In order to solve this problem, a dielectric filter 1' is suggested in which a through-hole 3 has a large-diameter hole portion 3a and a small-diameter hole portion 3b, which communicates with the large-diameter hole portion 3a, as shown in FIGS. 10 and 11, instead of the pattern electrode 7 disposed on the top surface of the dielectric block 2 as in the dielectric filter 1 shown in FIG. 9 (for example, see the translated PCT international publication No.8-512187). In the dielectric filter 1', since a pattern electrode is not required to be formed on the dielectric block 2, there is a solution of the problem involving the dielectric filter 1 shown in FIG. 9, that is, fluctuations in the coupling capacitance and the like due to the bleeding associated with the printing. This results in facilitating the miniaturization of the dielectric filter.
In the dielectric filter 1', the coupling capacitance Ck and the stray capacitance Cs of the dielectric resonators 6 which are disposed adjacent to each other can be adjusted by changing the diameter of the large-diameter hole portion 3a of each through-hole 3. However, when the length L1 in the axial direction of the large-diameter hole portion 3a is large, the amount of the change in the coupling capacitance corresponding to changes in the diameter of the large-diameter hole portion 3a is extremely large. Therefore, fine-adjustment of the coupling capacitance Ck is difficult which causes a problem of a wide range of variation of the coupling capacitance Ck.
Although in the dielectric filter 1', the axial length of the through-hole 3 can be shortened by changing the axial length L1 of the large-diameter hole portion 3a and the axial length L2 of the small-diameter hole portion 3b, it is also difficult to optimally adjust the shortening, the coupling capacitance Ck, and the stray capacitance Cs, simultaneously. This results in a problem of a small degree of freedom in the design.